Heater/blower unit with load control

ABSTRACT

A substantially constant load impedance electrical heater has been provided. The heater is comprised of two sections, a first heater and a second heater. The first heater is continuously “on” to provide the bulk of the heat. A second heater is selectively engaged to provide slightly more heat to the medium, heating the medium to the desired temperature. The output temperature of the heated medium is maintained by controlling the duty cycle of the second heater. More drastic changes in temperature are accomplished by changing the power continuously dissipated by the first heater. The greatest heating control is obtained by defining the first and second heaters as subsets from a bank of selectable heater elements. A method of minimizing fluctuations in the loading of a high-wattage electrical device is also provided.

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to forced-air convection heatersand, more particularly, to a high-wattage electrical device where loadvariations in the powering of the device are minimized to prevent lightflicker.

[0002] The electrically loading of a high-power device often results inan instantaneous voltage droop in the associated power supply. Forexample, when the lights momentary dim as a refrigerator turns on. Theseinstantaneous load changes can even cause power surges which blow fusesor harm other electrical equipment on the power line. When the loadingof the mains supply by a high-power electrical device changes rapidly, anoticeable flicker in the electrical lighting can occur.

[0003] A variety of consumer electronics products produce large voltagefluctuations which, at the very least are annoying and, often timesdisruptive to computer equipment and monitors. Some of thesehigh-wattage electric heating devices include clothes irons, electricfrying pans, skillets, woks, fondue pots, waffle irons, toasters, hairdryers, portable heaters, and electric blankets.

[0004] The disruptive effect of high-wattage electrical devices is moreof a concern in medical settings. Electrical forced-air heaters areoften used in keeping patients warm during an operation. However,doctors have been known to have the heaters turned off, to avoid theobnoxious effect of light flicker.

[0005] A typical forced-air warming unit consists of a blower, heater,and a temperature controller. The temperature controller moderates thepower supplied to the heater so that the bulk temperature of the airexiting the warming unit, or at some other control point, is maintainedat a fixed, set-point value. In general, the warming unit's heater issized so that its power dissipation is much greater than required tomaintain a given air temperature. The large power dissipation permitsthe heater to meet steady-state thermal requirements within a wide rangeof ambient temperatures. Further, the time required to achieve theset-point temperature is minimized.

[0006] Several strategies are available for regulating the powersupplied to the heater, and one of the most common is known aspulse-width modulation. Pulse-width modulation works by supplying thefull supply voltage to the heater as a square wave. The duty cycle (theratio of the on-time to the complete period) is varied by the controllerso that the power supplied to the heater, averaged over time, maintainsthe set-point temperature.

[0007] One problem associated with the pulse-width modulation techniqueis the potential for extremely severe, periodic power mains loading,which occur with every transition to an “on” cycle by the controller. Ina typical warming unit, the entire heater load (approximately 0.8-1.2kW) is switched on and off at a duty cycle which is proportional to theproduct of mass airflow and the required temperature difference.Switching a load of this magnitude causes a large inrush of current toflow in the power mains. Because of the power line impedance, thevoltage on the power mains drops when these large current inrushesoccur. This voltage drop can cause a perceptible flicker in any lightconnected to the same mains as the warming unit.

[0008] Several techniques are known which may be used to minimize theflicker. However, these methods all have certain drawbacks which makethem unsuitable in some respect.

[0009] One technique involves reducing the switching frequency below 0.2Hz (one transition every 5 seconds, or longer). This switching frequencyappears to be a threshold below which most people do not perceiveflicker. However, since the switching period is very long, it is notpossible to maintain the air temperature of the warming unit within anacceptable range.

[0010] Another technique involves switching power to the heater load ata rate equal to the line frequency. This technique requires specializedcircuitry which synchronizes the switching rate to that of the appliedline frequency, typically between 50 and 60 Hz. This method is veryeffective at eliminating flicker. However, because of the relativelyrapid current transition rate, this technique also generates a largeamount of electromagnetic emissions which must be suppressed withexpensive and massive filtering circuitry.

[0011] It would be advantageous if a high-wattage heating device couldbe developed that would minimize load fluctuations upon the powersupply.

[0012] It would be advantageous if forced-air heating units could bedeveloped which did not produce a noticeable flicker in the lighting. Itwould be advantageous if this heater were available for use in hospitalsettings.

[0013] It would be advantageous if a “flicker-free” heater could bedeveloped that was capable of operating over a wide temperature range,and also capable of rapidly responding to the selection of a newset-point temperature, or a change in input temperature.

SUMMARY OF THE INVENTION

[0014] Accordingly, a convection heater is provided having asubstantially constant load to minimize light flicker. The heater iscomprised of two basic sections, a roughing (first) heater and afinishing (second) heater. The first heater uses and dissipates most ofthe power, continuously heating the air to a first temperature which isclose to the desired output temperature. The second heater variablyheats the air. The combination of the first and second heaters raisesthe air temperature from the first temperature to the desiredtemperature. In this manner, the changes in the heater loading remainrelatively small.

[0015] Typically, the power dissipated by the first heater is at leasttwice as great as the second heater. However, the critical feature isthat the peak power of the second heater is minimized, for example, to apeak power of less than 200 watts. The variable loading of suchlow-power element produces no noticeable light flicker.

[0016] In the simplest aspect of the invention, the first heater is asingle element and the second heater is a single element. In someaspects of the invention the first heater is multitapped. Upondetermination of the desired output temperature, the first heater stageis selected which closely approaches, but does not exceed, theset-point. The second heater is then used to make up the differencebetween the heat supplied by the first heater and the desired outputtemperature. In some aspects of the invention, the first heater stagesare dynamically varied to more quickly approach the target temperature,and the minimize the difference in heat that must be applied by thesecond heater.

[0017] In one aspect of the invention, the first and second heaters areboth comprised of a plurality of heater sections. That is, the firstheater is a first combination of heater sections selected from theplurality of heater sections, and the second heater is a secondcombination. Each of the plurality of heater sections dissipates adifferent peak power level, where the difference is graduated in stepsof less than approximately 200 watts. Depending on the desired operatingtemperature and ambient conditions, the heater controller selectivelyactivates each of the heater elements. A first peak power is generatedby the first heater throughout a timed cycle, and a second peak powerlevel intermittently occurs during the cycle. The first and second powerlevels are dynamic, so that the absolute values of the first and secondpeak powers may change with every cycle.

[0018] A method for regulating the loading of a high-wattage powerdevice is also provided. The method comprising:

[0019] continuously dissipating a first peak power into a medium; and

[0020] periodically dissipating an additional peak power into themedium.

[0021] The additional peak power is selected to be small. As a result ofdissipating the first peak power, an output temperature is generatedthat is approximately the desired medium temperature. As a result ofperiodically adding an additional peak power, the desired temperature isgenerated.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIGS. 1a and 1 b are diagrams illustrating the two-element aspectof the present invention convection heater, having light flickersuppression characteristics.

[0023]FIG. 2 illustrates exemplary first duty cycle patterns used tocontrol the enablement of the first and second heaters.

[0024]FIG. 3 is a block diagram schematic illustrating a multitappedfirst heater aspect of the invention.

[0025]FIG. 4 is a block diagram schematic of a controller of the presentinvention.

[0026]FIG. 5 illustrates a multistage aspect of the present inventionheater.

[0027]FIG. 6 is a flowchart illustrating a method for heating inaccordance with the present invention.

[0028]FIG. 7 illustrates a detailed aspect of the method described byFIG. 6.

[0029]FIG. 8 illustrates a selectable first peak power aspect of themethod described by 7.

[0030]FIG. 9 illustrates a dynamic first and second peak power aspect ofthe method described by FIG. 8.

[0031]FIG. 10 is a flowchart illustrating a method for controlling athree-element heater with a ten-bit control word.

[0032]FIG. 11 is a diagram illustrating the ten-bit control word.

[0033]FIG. 12 is a flowchart illustrating an alternate aspect of themethod of the present invention for minimizing fluctuations in powerconsumption.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0034]FIGS. 1a and 1 b are diagrams illustrating the two-element aspectof the present invention convection heater, having light flickersuppression characteristics. In FIG. 1a, a heater 10 comprises a firstheater 12, to continuously heat a medium to a first temperature. Theentry of the unheated medium into heater 10 is represented by the arrowlabeled with reference designator 14. A second heater 16 intermittentlyheats the medium, in combination with the first heater 12, to a secondtemperature, greater than the first temperature. The passage of themedium at the second temperature, from the heater 10, is represented bythe arrow labeled as reference designator 18.

[0035] The invention is basically realized by placing at least twoindependent heaters within the same airflow, medium, or fluid stream.The first heater, referred to as the roughing heater 12, operatescontinuously, and is sized so as to provide enough power to raise thetemperature of the input medium stream 14 to within a few degrees of adesired set-point value. The second heater, referred to as the finishingheater 16, is a relatively low-power heater. During normal operation,both heaters 12 and 16 are activated, but only the power of secondheater 16 is modulated to maintain the desired medium outlettemperature.

[0036] The first heater 12 dissipates a first peak power. Thecombination of the first heater 12 and the second heater 16 dissipates asecond peak power, greater than the first peak power. The second heater16 dissipates a third peak power, which is the difference between thefirst and second peak powers. As the power consumed by the second heater16 is relatively small, the periodic loading of the power mains, as wellas the attendant voltage drop, is minimized. In some aspects of theinvention the first heater 12 can dissipate twice, three times, or evenmore, power than the second heater 16. More critical, however, is thefact that the second heater 16 dissipates less than approximately 200watts of peak power. In some aspects of the invention, the second heaterdissipates less than approximately 130 watts of peak power. In a typicalaspect of the invention heater 10 is binary; binary because there areonly two heating elements, and because the first element 12 is twice thepeak power level of second element 16.

[0037] A controller 20 has an output on line 22 to provide temperaturecontrol commands. The second heater 16 has an input on line 24 connectedto the controller 20. The second heater 16 varies the intermittentdissipation of power in response to commands from the controller 20.

[0038] The controller 20 has an input on line 24 to receive temperatureinformation. A first temperature sensor 26 measures the secondtemperature 18 at the output 27 of heater 10. The second temperature istypically the heater output temperature, however, the second temperaturemay also represent the medium temperature at some other control point inthe heater. The first temperature sensor 26 has an output on line 24,connected to the input of the controller 20, to provide secondtemperature information. The controller 20 provides temperature controlcommands on line 22 in response to the second temperature information.The second heater 16 varies the intermittent dissipation of power inresponse to temperature control commands from the controller 20.

[0039] In the context of the typical forced air warming (FAW)application, the heat demand remains nearly constant at steady-stateconditions, so the second heater 16 is designed to supply a smallfraction of the total required power. For example, an ambienttemperature of 20° C. is assumed, with an output (second) temperature of43° C. being desired. The FAW unit delivers air at a constant 30 ft³/m(14.2 L/s). If, for simplicity, no extra heat losses are assumed, thenthe power required to raise the air temperature 23° C. is given by

Q={dot over (m)}c _(p) ΔT

[0040] where$\overset{.}{m} = {{\left( {1.12\quad \frac{kg}{m^{3}}} \right)\left( {14.2\frac{L}{s}} \right)\quad \left( {0.001\frac{m^{3}}{L}} \right)} = {0.016\frac{kg}{s}}}$and${\overset{.}{m}c_{p}} = {{\left( {0.016\frac{kg}{s}} \right)\left( {1.008\quad \frac{kJ}{{kg} - K}} \right)} = {0.016\quad \frac{kJ}{s - K}}}$so$Q = {{\left( {0.016\frac{kJ}{s - K}} \right)\left( {23K} \right)} = {{0.368\quad \frac{kJ}{s}} = {368W}}}$

[0041] The first heater 12 can be sized to supply 320 W (87% of thetotal) and the second heater 16 to supply 100 W (114% of total incombination). In this case, the first heater 12 would raise the airtemperature 20.0 K (°C.) to 40° C., and the smaller heater 16 would haveto make up the difference. In practice, the heat losses are notinsignificant and both heaters would probably be larger than computedhere to meet the full range of air flow, ambient temperature and linevoltage input conditions, and there is inevitably some trade-off betweenperformance and flicker suppression. To meet the full range oftemperatures, the first heater 12 must be sized to meet the restraintsimposed by maximum voltage, ambient air temperature, and low air flow,and yet have enough power to control the temperature at low voltageinput, low ambient temperature, and high air flow. If a higher powersecond heater is required, the flicker suppression characteristics ofthe heater may be degraded. More sophisticated embodiments of thepresent invention, presented below, address this problem.

[0042] The peak power dissipated by the heaters 12 and 16 varies withrespect to the application. In general, no more than about 200 W isswitched to avoid a perceptible flicker. In a typical FAW unit, theinput power requirement is about 500-600 W at steady-state conditions.If the first heater 12 is 500 W, then its current is I=P/E=500/120=4.2A. Its resistance at 120 VAC is R−E/I=120/4.2=28.5 ohms. Because of lineimpedance, a value of 35 ohms is often used. The second heater's 16values are approximately 150 W and 96 ohms. The average power, as statedabove, is about 600 Watts for an average environment. The peak power maygo as high as 1000 Watts for a properly sized heater combination.

[0043]FIG. 2 illustrates exemplary first duty cycle patterns used tocontrol the enablement of the first 12 and second 16 heaters. Thecontroller 20 provides temperature commands on line 22 (see FIG. 1a),which correspond to a first duty cycle of width-modulated pattern ofpulses. As shown in the “steady state” scenario, the second heater 16 isenabled by selectively connecting heater 16 to an electrical powersource (not shown). Therefore, the second heater 16 dissipates power inresponse to the first duty cycle pulse-width modulation, and the secondtemperature 18 is responsive to the modulation of the pulse widths. Itshould be noted that the proportional time devoted to the engagement ofthe second heater 16 is approximately equal between cycles 1 and 2 insteady state, as shown. However, the division of the cycle between thefirst 12 and second 16 heaters may vary from cycle to cycle. That is,the width modulation may vary, as shown in the “dynamic” scenario.

[0044] In FIG. 1b a heater hose 28 is shown to deliver the heated medium18. Hose 28 has a proximal end 29 attached to the output 27 of heater 10and a distal end 30 to deliver the heated medium 18 to a target. In someaspects of the invention first temperature sensor 26 is located at thehose proximal end 29, as shown in FIG. 1b, to reduce complexities in theconstruction, use, and maintenance of hose 28. Then, heater 10 has aninput 31 to accept the medium to be heated. The input temperature of themedium is defined as the third temperature. A second temperature sensor32 measures the third temperature. The second temperature sensor 32 hasan output on line 33 connected to the controller 20 to provide thirdtemperature information. The controller 20 varies the temperaturecontrol commands on line 24 in response to the third temperatureinformation, and second heater 16 varies the power dissipated inresponse to the temperature control commands from the controller 20.Alternately, first heat sensor 26 is mounted at the distal end 30 ofhose 28 (not shown), and the second temperature sensor 32 isunnecessary. Although hose 28 is often used with heater 10, especiallyin medical applications, it is not shown in the following aspects of thepresent invention in the interest of clarity.

[0045]FIG. 3 is a block diagram schematic illustrating a multitappedfirst heater 12 aspect of the invention. Since the warming unit mustproduce heated air at several discrete exhaust temperatures and operatewithin a fairly large range of ambient input temperatures, a singlefixed-value first heater 12 is not always adequate. In some aspects ofthe invention, the first heater 12 includes a multitapped, multistagearrangement, or plurality of elements, represented by elements 12 a, 12b, and 12c. The appropriate section (or sections) 12 a, 12 b, or 12 c,is activated in response to the selection of a discrete set-pointtemperature. The resistance and power dissipated by element 12 c istypically greater than the resistance of element 12 b. Likewise, theresistance of element 12 b is typically greater than heater element 12a. Alternately, the elements 21 a, 12 b, and 12 c may all dissipateequal peak power levels, so that variations in the first peak powerresult from the engagement of multiple elements simultaneously.

[0046] The multitapped, or multistage, elements 12 a, 12 b, and 12 cselectably provide a first peak power from a range of peak power values.The multitapped first heater 12 is represented herein with threesections 12 a, 12 b, and 12 c, but the invention is not limited to anyparticular number of sections. In this aspect of the invention, thefunction of the second heater section 16 remains substantially the sameas explained in the description of FIG. 1.

[0047] Alternately stated, the first heater 12 provides a selectablesteady-state first peak power level responsive to elements 12 a, 12 b,and 12 c. The first heater 12 has inputs on lines 34 a, 34 b, and 34 c,corresponding respectively to elements 12 a, 12 b, and 12 c, to receivefirst temperature selection commands to select the first peak power. Thecontroller 20 has an output connected to the first heater 12 input onlines 34 a, 34 b, and 34 c to provide temperature control commands. Thefirst heater 12 dissipates a selected steady-state first peak power inresponse to temperature control commands from the controller 20.

[0048] The first heater sections 12 a, 12 b, and 12 c are selectable bya mechanical or electrical switch. Alternately, the first heatersections (as shown) are switched by controller 20 through lines 34 a, 34b, and 34 c. In the simplest aspect of the invention the first heaterelement(s) remain constantly engaged, once selected. However, in otheraspects of the invention the first heater elements 12 a, 12 b, and 12 care varied to optimally produce the desired medium output temperaturewith the minimum of light flicker. Then, the selection of the firstheater sections 12 a, 12 b, and 12 c is dynamic.

[0049] The highest power heater element is selected which will not heatthe medium beyond the desired temperature, and the medium outputtemperature is monitored. As above, the first heater remains “on” forthe duration of the operation, with a dynamic selection of the firstheater element occurring at a relatively slow rate, for example, lessthan one change every 10 seconds. The first heater is responsive tocommands from controller output lines 34 a, 34 b, and 34 c which providecontroller commands which vary the outputs with respect to time. Thesecond heater 16 is still controlled by line 22, as described above, toachieve the desired set-point temperature. Alternately stated, the firstheater is always on, producing a constant first peak power level.However, the first heater (first peak power level) is allowed to varyonce a cycle, with the second heater intermittently being engaged insidethat cycle.

[0050] A set-point, or desired, temperature is selected by set-pointmechanism 35, connected to the controller 20 on line 36 to supplytemperature information used in controlling the second temperature. Thisscheme allows the second heater 16, and thus, the amount of powerswitched by the controller 20 into second heater 16, to remainrelatively constant and small regardless of the desired outlettemperature 18.

[0051] In one aspect of the invention the controller 20 uses acalculation of the derivative of the second temperature information toprovide temperature control commands, which select the proper firstheater elements 12 a, 12 b, and 12 c. Information derived from set-pointmechanism 35 and first sensor 26 are used in this calculation.

[0052] Heater 10 also comprises an electrical power source 37 connectedto the controller 20 on line 38. A line voltage sensor 39 measures theline voltage of the electrical power source 37 on line 38. The linevoltage sensor 39 has an output on line 40 connected to an input of thecontroller 20 to provide voltage information.

[0053] The controller 20 provides temperature control commands on lines34 a, 34 b, and 34 c which selectively connect the electrical powersource 37 to the heater sections 12 a, 12 b, and 12 c, respectively. Theheater sections 12 a, 12 b, and 12 c dissipation of power is controlledin response to the temperature control commands.

[0054] The controller 20 provides temperature control commands generatedby a proportional-integral-derivative (PID) formula responsive totemperature variables input to the controller 20. The temperaturevariables include the information from sensor 26 (medium output, orsecond temperature 18), the second sensor 32 (medium input, or thirdtemperature 14), line voltage sensor 39, and the set-point temperaturefrom mechanism 35. Heater sections 12 a, 12 b, and 12 c vary the powerdissipated in response to the temperature control commands from thecontroller 20.

[0055] There are several ways in which the algorithm is supplied withinput information. One method involves using the line voltage, inputtemperature, and desired outlet temperature values to select theappropriate heater combinations. This method uses either a look-up tableor a transfer function to accomplish its selection.

[0056] Another control algorithm selects the appropriate first andsecond heaters based on a determination the derivative of the outlettemperature. This method has a theoretical advantage over the firstmethod in that it does not require the value of the input temperature asone of the input values.

[0057]FIG. 4 is a block diagram schematic of a controller of the presentinvention. The controller accepts inputs from sensors, such as firstsensor 26, second sensor 32, line voltage sensor 39, and set-pointmechanism 35. A microprocessor 50 evaluates the sensor input and outputstemperature control commands on lines 52, 54, 55, and 56. Power supplycurrent enters controller 20 (or is otherwise controlled) from powersource 37 on line 38. The power lines 38 are isolated with electricalisolators 57, 58, 59, and 60, or provided by isolated power supplies(not shown) in some aspects of the invention. The control of lines 34 a,34 b, and 34 c (to heater sections 12 a, 12 b, and 12 c, respectively,see FIG. 3) is a result of signals on signal lines 52, 54, and 55,respectively. Likewise, second heater 16 is dynamically varied withrespect to time in response to a control signal on line 56. In someaspects of the invention a storage medium 61 is in communication withmicroprocessor 50 to provide program instructions and controlalgorithms. Alternately, storage medium 61 is embedded in themicroprocessor 50 (not shown).

[0058]FIG. 5 illustrates a multistage aspect of the present inventionheater. A novel method of minimizing the number of heater stages in thisinvention is realized by employing at least a two stage (or more)heater. FIG. 5 depicts a three-stage heater 10. In its simplest from, analgorithm selects the largest possible load, or first combination ofthree available heaters, for the first heater. The algorithm thenselects a second combination of the three heaters to act as the secondheater. This embodiment now permits both the first and second heaters tobe selectable to minimize loading changes. The preferred algorithm isany PID (Proportional Integral Derivative) loop algorithm.

[0059] A PID algorithm examines the temperature error from threeperspectives. A proportional analysis examines the difference betweenthe set-point temperature and the measured temperature. The result is adetermination of the fastest way of reaching the set-point temperature.An integral analysis examines the accumulation of error as equilibrium(the set-point temperature) is approached and achieved. It addssteady-state precision by counteracting low frequency error. Adifferential analysis examines the rate of change in the error. Itcounteracts the proportional calculation, to prevent overshoot,undershoot, and to dampen out ringing.

[0060] The invention of FIG. 5 is dynamic with respect to power levelsand modulation timing. For the purpose of clarity, the invention isfirst presented with the assumption that the power levels of the firstand second heaters sections remain constant. That is, the power level ofthe first and second heaters are not dynamic. However, as presented indetail below, the dynamic aspects of these heaters provide greatutility.

[0061] Heater 10 comprises a plurality of selectably connectable heaterelements 62, 64, and 66 controlled by lines 34 a, 34 b , and 34 c. Theoperation of the controller 20 and the use of lines 34 a, 34 b, and 34 cis similar to that described above in the explanation of FIGS. 3 and 4.Three sections 62, 64, and 66 are shown, but the present invention isnot limited to any particular number of heater sections. Further, theheater sections are represented by resistive elements, but the conceptof the present invention is applicable to many other high-wattage powerconsuming devices. The concept of the first and second heater becomesmore conceptual in the more sophisticated control aspects of theinvention. The heater 10 operates in a first phase, with a firstcombination of heater sections. A second phase, representing theaddition of a second heater to the first heater, is represented by asecond combination of heater sections. That is, the first phase heateris selected from the selectably connectable plurality of heater elements62, 64, and 66. Likewise, the second phase heater, the combination offirst and second heaters, is selected from the selectably connectableplurality of heater elements 62, 64, and 66. Typically, the differencein peak power between the first and second phase heater combinations isselected to be as small as possible, to minimize the change in loading.

[0062] The first heater element 62 dissipating a peak power, a secondheater element 64 dissipating a peak power greater than that of thefirst heater element 62, a third heater element 66 dissipating a peakpower into the medium, greater than that dissipated by the second heaterelement 64, and an option of selecting no heater (dissipating no peakpower). Then, the first and second heaters are selected from the groupconsisting of the first 62, second 64, third 66 heater elements, andcombinations of the first heater element 62, second heater element 64,third heater element 66, and no heater element. As above, thecombination of second and first heaters dissipates a second peak powergreater than the first peak power dissipated by the combination of firstphase heaters. The difference between phases, with the enablement of thesecond heater, is a third peak power of less than approximately 200watts.

[0063] For example, the first phase heater may consist of the first andsecond heater sections 62 and 64 to generate a first peak power. Turningon the second heater, in addition to the first heater, generates thesecond peak power (the second heater phase), and entails finding thenext largest combination of heater sections. In this case, first andsecond heater sections 62 and 64 are turned off, and third heatersection 66 is turned on to generate the second peak power.

[0064] Typically, the peak power of the third heater element 66 isapproximately twice as great as the peak power of the second heaterelement 64. The peak power of the second heater element 64 isapproximately twice as great as the peak power of the first heaterelement 62. This same relationship holds true when four, or more, heatersections are used.

[0065] For simplicity, the heater of FIG. 5 has been described as havingfirst and second peak power values that remain constant, once chosen.However, the first and second peak power values may also varydynamically. Alternately stated, the first and second combinations ofthe plurality of heater elements 62, 64, and 66 vary dynamically withrespect to peak power and time. In the dynamic change scenario, thefirst peak power remains on continuously and the additional third peakpower is intermittently added to the first peak power, as before. Thethird peak power level is less than 200 watts. But now the value of thefirst and third peak power levels may change every cycle. Returningbriefly to FIG. 2 to discuss the “dynamic” scenario, cycle 1 begins witha second phase (first plus third peak power) and ends with a first phase(first peak power only). The second phase of cycle 2 may be at adifferent power level than that of cycle 1. That is, the first peakpower may change.

[0066] For example, in cycle 1 the second peak power (first plus thirdpeak powers) is 550 watts and the first peak power level is 400 watts.In cycle 2 the second peak power is 450 watts and the first peak poweris 300 watts. The power changes from 550 watts, to 400 watts in cycle 1,and from 450 watts, to 300 watts in cycle 2. None of the changes inpower are greater than 200 watts, including the changes between cycles.Note, the timing relationship between the second and first phases remaindynamic also, as the amount of “on” time of the second and first phaseschanges between cycle 1 and cycle 2.

[0067] As above, the controller 20 provides temperature control commandsgenerated by a proportional-integral-derivative (PID) formula responsiveto the controller inputs. The plurality of heater elements 62, 64, and66 vary the power dissipated in response to the temperature controlcommands from the controller 20. The temperature control commandsprovided by the controller 20 include a two-part digital word. Eachheater of the plurality of heaters 62, 64, and 66 is selected inresponse to a bit in the first part of the digital word, and theintermittent occurrence of the second peak power is responsive to thesecond part of the digital word. A more detailed example of adynamically changing first/second phase heater is presented below in thedescription of FIG. 10.

[0068] Referring to FIG. 5, in many applications the medium to be heatedis air, although the concept of the present invention is more farreaching. Then, a fan or blower 80 is provided. The blower 80 deliversair to the heater sections 62, 64, and 66.

[0069] Although one intent of the invention is to eliminate flickerwithin the operating room during the operation of a convective warmingunit, the invention has much wider application. In particular, the basicinvention could be used with any type of high-power electric heater,irrespective of heat transfer mode employed by the device. Other medicaldevices, such as fluid warmers, mattress-type circulating-fluid patientwarmers, neonatal (over-the-bed) radiant warmers, and feet warmers(portable heaters) can also be made “flicker-free” with the presentinvention.

[0070]FIG. 6 is a flowchart illustrating a method for heating inaccordance with the present invention. Referring briefly to FIG. 4, thismethod can be enabled using a software program including a set ofinstructions hosted in storage medium 61, which are carried out bymicroprocessor 50. Although the steps are numbered for a clearerpresentation of the process, no order should be inferred from thenumbering unless explicitly stated. Step 100 provides a medium, such asair, to be heated. Step 102 continuously dissipates a first peak powerinto the medium. Step 104 intermittently dissipates an additional peakpower, in combination with the first peak power, into the medium. Step106 is a product, a medium heated by a combination of steady-state andpulsing heaters.

[0071] The most basic mechanism of temperature control is in thevariation of the first duty cycle associated with the intermittentadditional peak power. That is, Step 104 includes varying theintermittence of the additional peak power. However, since the processis designed to be used for a number of input and output temperatures,the peak power levels must typically vary, as well as the intermittenttiming. Then, Step 102 includes selecting the first peak power from aplurality of peak power levels. Making the first peak power adjustablein Step 102, permits the power fluctuations associated with Step 104 tobe minimized. The combination of the first peak power and theintermittent peak power is defined as the second peak power, and theintermittent peak power is defined as the third peak power level, andStep 104 includes selecting the second peak power so as to minimize thethird peak power. Typically, Step 104 includes the third peak powerbeing less than approximately 200 watts.

[0072] Some aspects of the invention comprise further steps. Step 105 a,in response to Step 102, generates substantially a desired temperaturein the medium. Step 105 b, in response to Step 104, generates an outputtemperature that is the desired medium temperature.

[0073]FIG. 7 illustrates a detailed aspect of the method described byFIG. 6. A further step, Step 105 c, measures temperature variables.Specifically, Step 105 c measures the desired temperature over time.Step 104 includes varying the intermittence of the second peak power inresponse to the temperature measured in Step 105 c.

[0074]FIG. 8 illustrates a selectable first peak power aspect of themethod described by FIG. 7. Step 102 includes the first peak power levelbeing selectable, and Step 105 d selects the first peak power level inresponse to changes in the desired temperature over time measured inStep 105 c.

[0075] In some aspects of the invention the first peak power isdynamically selectable. A controller is provided in Step 100, along witha power supply to power the plurality of heating elements. Step 105 c,measures the power supply voltage, output medium temperature, theset-point, and the ambient medium temperature. Then, Step 105 d includesthe controller selecting a first peak power levels in response to themeasurement of the power supply voltage, ambient medium temperature,set-point, and output medium temperature measured in Step 105 c. Theselection of the second peak power in Step 104 automatically followsfrom the selection of the first peak power. Further, Step 104 includesvarying the intermittence of the second peak power level in response tothe measurement of the variables in Step 105 c.

[0076]FIG. 9 illustrates a dynamic first and second peak power aspect ofthe method described by FIG. 8. Step 100 provides a plurality of heatingelements. Then, Step 102 includes selecting a first combination ofheating element from the plurality of heating elements to generate thefirst peak power, and Step 104 includes selecting a second combinationof heating elements from the plurality of heating elements to generatethe second peak power. When the second peak power level is 200 watts, orless, it is possible for the first peak power to be zero. That is, thefirst combination of elements can be the selection of no elements.

[0077] Typically, Step 100 provides at least first, second, and thirdheating elements, although the concept is applicable to more heatingelements. The third heating element dissipates a peak power greater thanthat dissipated by the second heating element, and the second heaterdissipates a peak power greater than that dissipated by the firstheating element. Step 102 includes generating the first peak power fromheating elements selected from the group consisting of the first,second, and third heating elements, or no heating element. Step 104includes generating the second peak power from heating elements selectedfrom the group consisting the first, second, and third heating elements.As above, Step 104 includes the third peak power being less thanapproximately 200 watts.

[0078] Step 100 provides that the plurality of heating elements areselectively connectable to a power supply voltage. Step 105 f selectablyconnects the plurality of heating elements to the power supply inresponse to the generation of the temperature control commands by thecontroller. Step 102 includes selecting from the plurality of heatingelements to generate a first peak power level in response to thetemperature control commands. Step 104 includes selecting from theplurality of heating elements to generate a second peak power level, andvarying the intermittence of the second peak power in response to thetemperature control commands.

[0079] Since the second peak power level is typically the next largestincrement of power available, the selection of the second power levelautomatically follows from the selection of the first power level.Therefore, the selection of the first peak power level, is directlyrelated to the selection of the second peak power. Alternately, thefirst peak power level could follow from the selection of a second peakpower level.

[0080] Specifically, in Step 105 f a two-part digital word is created,where the first part includes a plurality of bits, with each bitcorresponding to a heating element in the plurality of heating elements.The second part of the digital word creates a timing pattern. Then, Step102 includes selecting heating elements in response to the first part ofthe digital word. Step 104 includes varying the intermittence of thesecond peak power level in response to the second part of the digitalword. The first peak power is constantly maintained, while the thirdpeak power is intermittently added to the first peak power. However, theactual value of the first peak power level (and therefore the third peakpower level) may be dynamically adjusted.

[0081] Typically, Step 100 provides that the controller has a pluralityof inputs. Step 105 c includes providing temperature related variablesto the controller including the medium output temperature and thedesired temperature set-point. Step 105 f generates the temperaturecontrol word in response to the temperature related variable using aproportional-integral-derivative (PID) algorithm.

[0082]FIG. 10 is a flowchart illustrating a method for controlling athree-element heater with a ten-bit control word. In Step 200 thecontroller is started, and a program is loaded in Step 202. A desiredmedium, or second temperature, is entered in Step 204. In Step 206 thecontroller reads the set-point temperature entered in Step 204. In Step208 the output sensor data is read. The sensor data is supplied in Step210.

[0083] In Step 212 the heater elements are selected and the timing ofthe first and second heaters is calculated. In Step 214, thecalculations are converted into a ten-bit control word, with the threemost significant bits (first partial word) set for the second powerlevel. Step 216 generates a first partial word to activate the firstpower level from the plurality of heater elements, with the remainder ofthe word being used for timing, the proportional “on” time of the secondpower level with respect to the first power level.

[0084]FIG. 11 is a diagram illustrating the ten-bit control word. Thefirst part of the digital word includes three bits, where each bit isused to control the activation of a heater section, and so control thepeak power dissipated. In the example of FIG. 11, the first part has avalue of 4 (100). The “1” bit enables the third heating element. The twozeros disable the first and second heating elements. In the generationof the digital word first part, a first plurality of bits is generated,in this case three, with each bit corresponding to a heating element.One bit is used for each heating element. If four heating sections areused, the first part of the digital control word would be four bits.Alternately, the first part of the word (the heater control bits) may belocated in other bit locations inside the 10-bit word, however,alternate locations inside the word may require a separate step ofshifting which is avoided in the present scheme.

[0085] The peak power time duration is responsive to the digital wordsecond part. To control the heater accurately, timing control is brokendown into half power cycles, where one power cycle is {fraction(1/60)}^(th) of a second in North America, and {fraction (1/50)}^(th) ofa second in the rest of the world. For example, in North America thereare approximately 120 half cycles per second. The generation of thedigital word second part includes a second plurality of bits, in thiscase seven, the sum of which defines a timing pattern. The seven leastsignificant bits of the second part of the digital word are used tocreate 128 incremental steps, where 120 of the steps evenly divide asecond. In the example shown in FIG. 11, the third heater element isenabled for a count, or time duration of {fraction (16/128)}, or⅛^(th)of a second (0010000). The the cycle, or ⅞^(th)of a second. Thefirst part of the cycle is the second phase including both the first andsecond heaters (second peak power). Since ⅛^(th)of a second is 15 halfcycles, from counts 16 to 120 the first heater is enabled (first peakpower), which is the next lower increment in power dissipation.

[0086] The heater elements are graduated by approximately equal powerdifferences, where each difference is less than 200 watts. The firstheater in this example would be the enablement of both the first andsecond heating elements, while the third heating element is disabled.That is, a first word part becomes (011) in the first heater phase ofthe heating cycle. Alternately stated, the selection of a peak powerdissipation includes varying the power between a first and second peakpower, and the selection of a peak power time duration includesselecting a first duration for the first peak power and a secondduration for the second peak power. The difference between the first andsecond peak power levels is less than 200 watts.

[0087] The medium temperature to be controlled is sensed by a suitablesensor in Step 210, and after amplification, is fed into an Analog toDigital converter. The digital value of this temperature is processedthrough any control scheme including any PID control scheme or even justa proportional scheme. An output word is generated which is indicativeof the level of required power.

[0088] Typically, each heater section dissipates twice the peak power ofthe next lower heater section. The corresponding power value of theseheaters is represented in binary format. If the third position from lefthas a value of “x Watts”, the second position has the value “2x Watts”,and the first position has a value “4x Watts”. The right 7 positionsprovide the digital value of duty cycle, which are explained in moredetail, below.

[0089] Assuming the unit is started cold, the digital word will be:

111 (1111111)  second heater phase

[0090] where the bold number represents the first part of the digitalword, and the number in parenthesis represents the second part of theword. All three heaters are on with the duty cycle of 128 out of 128.The first heater phase is “0” duty cycles. The selection of the first(or second) peak power includes engaging each heating element inresponse to the corresponding bit in the digital word first part. Up tothis point in discussion, the present invention has been presented as aconstant dissipation of a first peak power and the intermittentdissipation of a second, greater, peak power. However, as presented inthe example of FIG. 10, the invention can also be embodied as adissipation of a second peak power with intermittent reductions in powerto a first peak power level.

[0091] As the heater warms up, less power is required, and the dutycycle is decreased.

111 (1110111)  second heater phase

[0092] All heaters are on for {fraction (119/128)}^(th) of a second and:

110   first heater phase

[0093] the smallest heater is off for {fraction (9/128)}^(th) of asecond.

[0094] The first three bits of the control word are the heater controlbits, which exit the controller on individual I/O lines to controlseparate heaters. The rest of the control word, the right 7 bits areretained internally to indicate the duty cycle (the timing of the leftthree bits).

[0095] As the warming continues:

111 (0000001)  second heater phase

[0096] All heaters are on for {fraction (1/128)}^(th) of a second and:

110   first heater phase

[0097] the smallest heater is off for {fraction (127/128)}^(th),seconds.

[0098] In the next step of reduction:

111 (0000000)  second heater phase

[0099] The smallest heater is always off. There is no second half tothis cycle.

[0100] In the following steps different heater combinations are used.Both portions of each cycle are now shown on the same line. As warmingcontinues:

110 (1111111)  second heater phase

[0101] Meaning power level 6 is always on. As warming continues: SECONDFIRST 110 (0000001) 1/128^(th) sec. 101 127/128^(th) sec. 101 (1111111)

[0102] Meaning bits 4 and 1 are on continuously. The number indicated inthe parentheses corresponds to the portion of time the higher powersetting is on. 128 minus this number corresponds to the time the lowerpower setting is on. (0000000) defines a condition where no portion ofthe higher power level (the power indicated in front of the parentheses)is on. Therefore, the lower power is on all the time (for a fullsecond).

[0103] To continue the example further: SECOND FIRST 101 (1111000)120/128^(th) sec. 100 8/128^(th) sec. 101 (1110111) 119/l28^(th) sec.100 9/l28^(th) sec. 101 (1110110) 118/128^(th) sec. 100 10/128^(th) sec.101 (0000001) 1/128^(th) sec. 100 127/128^(th) sec. 101 (0000000) none100 continuous 100 (1110111) 119/128^(th) sec. 011 9/128^(th) sec.

[0104] The 10 bits act as one contiguous word internal to themicroprocessor and in all calculations. The first three bits areseparated at the output to directly run the heaters, after somebuffer/isolation amplifiers, see control lines 52, 54, and 55 of FIG. 4.

[0105] Returning to FIG. 10, Step 218 outputs the three most significantbits (first word part) to enable the heating elements. As explainedabove in the description of FIG. 11, Step 220 checks the mostsignificant bit and Step 222 enables the third heating element if thebit is a “one”. Likewise, Step 224 checks the second most significantbit and Step 226 enables the second heating element if the bit is a“one”. Step 228 checks the third most significant bit and Step 230enables the first heating element if the bit is a “one”. Step 232 checksthe timing associated with the second word, and returns to Step 206 forthe generation of a new control word.

[0106]FIG. 12 is a flowchart illustrating an alternate aspect of themethod of the present invention for minimizing fluctuations in powerconsumption. Step 300 provides a medium. Step 302 dissipates a variablepeak power into a medium at a first duty cycle. Step 304, in response toStep 302, is the product of a medium heated to a constant temperature.

[0107] Step 302 includes alternately dissipating a first peak power, anda second peak power, greater than the first peak power. Step 302includes varying the first duty cycle, while Step 304 includesmaintaining the constant output temperature in response to varying thefirst duty cycle. Step 302 includes the difference between the first andsecond peak powers being less than 200 watts.

[0108] A method of minimizing power fluctuations in the loading of ahigh-wattage power device have been provided above. The embodimentsinclude a heater made from first heater and a pulsed second heaterelements. Variations of the invention include a multitapped firstheater. A multistage heater where the first and second heaters areselected from a plurality of possible heating elements, provides themost flexibility. Other variations and embodiments of the presentinvention will occur to those skilled in the art.

What is claimed is:
 1. A heater comprising: a first heater, tocontinuously heat a medium to a first temperature; and a second heater,to intermittently heat the medium, in combination with said firstheater, to a second temperature, greater than the first temperature. 2.The heater of claim 1 in which said first heater dissipates a first peakpower, in which said combination of first and second heaters dissipatesa second peak power, greater than the first peak power.
 3. The heater ofclaim 2 in which said second heater dissipates a third peak power, whichis the difference between the first and second peak powers, the thirdpeak power being less than the first peak power.
 4. The heater of claim3 in which said second heater dissipates less than approximately 200watts of peak power.
 5. The heater of claim 3 further comprising: acontroller having an output to provide temperature control commands; andin which said second heater has an input connected to said controller,said second heater varying the intermittent dissipation of power inresponse to commands from said controller.
 6. The heater of claim 5 inwhich said controller provides temperature commands at a first dutycycle rate of modulated pulse widths; and in which said second heaterdissipates power in response to the first duty cycle pulse-widthmodulation.
 7. The heater of claim 5 in which said controller has aninput to receive temperature information, and further comprising: afirst temperature sensor to measure the second temperature, said firsttemperature sensor having an output, to provide second temperatureinformation, connected to the input of said controller; in which saidcontroller provides temperature control commands in response to thesecond temperature information; and in which said second heater variesthe intermittent dissipation of power in response to commands from saidcontroller.
 8. The heater of claim 7 in which said first heater includesa plurality of elements to provide a selectable first peak power, saidfirst heater having an input to receive first temperature selectioncommands to select the first peak power; in which said controller has anoutput connected to said first heater input to provide temperaturecontrol commands; and in which said first heater dissipates a selectedfirst peak power in response to temperature control commands from saidcontroller.
 9. The heater of claim 8 in which said controller outputprovides first temperature control commands which vary with respect totime; and in which said first heater dynamically varies the first peakpower in response to said controller output temperature commands. 10.The heater of claim 9 in which said controller calculates the derivativeof the second temperature information and provides temperature controlcommands in response to a calculation of the derivative.
 11. The heaterof claim 9 in which said controller has a plurality of inputs, andfurther comprising: a second sensor, to measure the medium inputtemperature, having an output connected to the input of said controllerto provide temperature information; a set-point control to select thesecond temperature, said set-point control having an output connected toan input of said controller to supply temperature information; anelectrical power source having a line voltage; a line voltage sensor tomeasure the line voltage of said electrical power source, said linevoltage sensor having an output connected to an input of said controllerto provide voltage information; in which said controller providestemperature control commands in response to the information from saidfirst sensor, said second sensor, said set-point control, and said linevoltage sensor; and in which said plurality of heater elements vary thepower dissipated in response to temperature control commands from saidcontroller.
 12. The heater of claim 9 comprising: a plurality ofselectably connectable heater elements; in which said first heater isselected from a first combination of said plurality of heater elements;and in which the combination of said first and second heaters isselected from a second combination of said plurality of heater elements.13. The heater of claim 12 in which said plurality of selectablyconnectable heater elements includes: a first heater element dissipatinga peak power; a second heater element dissipating a peak power greaterthan that dissipated by said first heater element; a third heaterelement dissipating a peak power, greater than that dissipated by saidsecond heater element; in which said first heater is selected from thegroup consisting of no heater element, said first heater element, saidsecond heater element, and said third heater element; and in which saidsecond heater is selected from the group consisting of said first,second, and third heater elements.
 14. The heater of claim 13 in whichthe peak power of said third heater element is approximately twice asgreat as the peak power of said second heater element, and in which thepeak power of said second heater element is approximately twice as greatas the peak power of said first heater element.
 15. The heater of claim12 in which the combination of said first and second heaters dissipatesa second peak power no more than approximately 200 watts greater thanthe first peak power dissipated by said first heater.
 16. The heater ofclaim 12 in which said controller provides temperature control commandsgenerated by a proportional-integral-derivative (PID) formula responsiveto the controller inputs; and in which said plurality of heater elementsvary the power dissipated in response to the temperature controlcommands from said controller.
 17. The heater of claim 12 in which saidfirst heater first combination of said plurality of heater elementsvaries dynamically; and in which said second heater second combinationof said plurality of heater elements varies dynamically.
 18. The heaterof claim 17 in which the temperature control commands provided by saidcontroller include a two-part digital word; in which each heater elementof said plurality of heater elements is selected in response to a bit inthe first part of the digital word; and in which the intermittentoccurrence of the second peak power is responsive to the second part ofthe digital word.
 19. The heater of claim 7 in which said controller hasa plurality of inputs, and further comprising: a heater hose having aproximal end connected to the output of said heater, and a distal end todeliver the heated medium; a heater input to accept the medium, at athird temperature, to be heated; a second temperature sensor to measurethe third temperature of the medium at said heater input, said secondtemperature sensor having an output connected to said controller toprovide third temperature information; in which said first temperaturesensor is located at the proximal end of said heater hose; in which saidcontroller varies the temperature control commands in response to thethird temperature information.
 20. The heater of claim 1 wherein themedium is air, and further comprising: a blower to deliver air to saidfirst and second heaters.
 21. A method for heating a medium comprising:continuously dissipating a first peak power into the medium; andintermittently dissipating an additional peak power, in combination withthe first peak power, into the medium.
 22. The method of claim 21 inwhich the dissipation of the first peak power includes selecting thefirst peak power from a plurality of peak power levels.
 23. The methodof claim 21 in which the combination of the first peak power and theintermittent peak power is a second peak power, with the intermittentadditional peak power being a third peak power level, and in which theintermittent dissipation of additional peak power includes selecting thesecond peak power so as to minimize the third peak power.
 24. The methodof claim 23 in which the third peak power is less than approximately 200watts.
 25. The method of claim 23 further comprising: in response to thedissipation of the first peak power, generating substantially a desiredtemperature in the medium; and in response to the intermittentdissipation of an additional peak power, generating an outputtemperature that is the desired medium output temperature.
 26. Themethod of claim 25 further comprising: measuring the desired outputtemperature over time; and in which the intermittent dissipation of anadditional peak power includes varying the intermittence of the secondpeak power in response to the measured temperature.
 27. The method ofclaim 26 in which the dissipation of the first peak power includes thefirst peak power level being selectable, and includes selecting thefirst peak power level in response to changes in the desired outputtemperature over time.
 28. The method as in claim 27 wherein a powersupply is provided, and further comprising: measuring the medium inputtemperature; measuring the power supply voltage; selecting the desiredmedium output temperature; in which the dissipation of the first peakpower includes selecting from the plurality of heating elements togenerate a first peak power levels in response to the measurement of thepower supply voltage, the medium input temperature, the medium outputtemperature and the selection of the desired medium output temperature;and in which the intermittent dissipation of an additional peak powerincludes varying the intermittence of the second peak power in responseto the measurement of the power supply voltage, the medium inputtemperature, the medium output temperature, and the selection of thedesired medium output temperature.
 29. The method of claim 27 wherein aplurality of heating elements are provided; in which the dissipation ofthe first peak power includes selecting a first combination of theplurality of heating elements to generate the first peak power; and inwhich the intermittent dissipation of an additional peak power includesselecting a second combination of the plurality of heating elements togenerate the second peak power.
 30. The method of claim 29 wherein atleast first, second, and third heating elements are provided, whereinthe third heating element dissipates a peak power greater than thatdissipated by the second heating element, and the second heaterdissipates a peak power greater than that dissipated by the firstheating element, in which the dissipation of the first peak powerincludes generating the first peak power from heating elements selectedfrom the group consisting of the no heating elements, first, second, andthird heating elements, and in which the intermittent dissipation of anadditional peak power includes generating the second peak power fromheating elements selected from the group consisting the first, second,and third heating elements.
 31. The method of claim 30 wherein acontroller is provided, and wherein the plurality of heating elementsare selectively connectable to a power supply voltage, and furthercomprising: selectably connecting the plurality of heating elements tothe power supply voltage in response to the generation of temperaturecontrol commands by the controller; in which the dissipation of thefirst peak power includes selecting from the plurality of heatingelements to generate first peak power levels in response to thetemperature control commands; and in which the intermittent dissipationof an additional peak power includes selecting from the plurality ofheating elements to generate second peak power levels and varying theintermittence of the second peak power in response to the temperaturecontrol commands.
 32. The method of claim 31 in which the generationtemperature control commands includes creating a two-part digital word,where the first part includes a plurality of bits, with each bitcorresponding to a heating element of the plurality of heating elements,and where the second part of the digital word creates a timing pattern;in which the dissipation of the first peak power includes selecting fromthe plurality of heating elements to generate a first peak power levelsin response to the first part of the digital word; and in which theintermittent dissipation of an additional peak power includes varyingthe intermittence of the second peak power in response to the secondpart of the digital word.
 33. The method of claim 32 wherein thecontroller has a plurality of inputs, and further comprising: providingtemperature related variables to the controller including the mediumoutput temperature and the desired temperature set-point; and generatingthe temperature control word in response to the temperature relatedvariable using a proportional-integral-derivative (PID) algorithm.
 34. Amethod for minimizing fluctuations in power consumption comprising:dissipating a variable peak power into a medium at a first duty cycle;and in response to dissipating a variable peak power, heating a mediumto a constant temperature.
 35. The method of claim 34 in which thedissipation of a variable peak power includes alternately dissipating afirst peak power, and a second peak power, greater than the first peakpower.
 36. The method of claim 35 in which the dissipation of a variablepeak power includes varying the first duty cycle; and in which theheating of the medium includes maintaining the constant outputtemperature in response to varying the first duty cycle.
 37. The methodof claim 36 in which the dissipation of a variable peak power includesthe difference between the first and second peak powers being less than200 watts.
 38. A method for programmably controlling a heatercomprising: generating a digital word including a first part and asecond part; selecting a peak power dissipation in response to thedigital word first part; and selecting the peak power time duration inresponse to the digital word second part.
 39. The method of claim 38further comprising: providing a medium to be heated; measuring thetemperature of the heated medium; and generating the digital word inresponsive to the measurement of the heated medium.
 40. The method ofclaim 39 in which the selection of a peak power dissipation includesvarying the power between a first and second peak power; and in whichthe selection of a peak power time duration includes selecting a firstduration for the first peak power and a second duration for the secondpeak power.
 41. The method of claim 40 wherein a first plurality ofselectively engagable heating elements is provided; and in which thegeneration of the digital word first part includes a first plurality ofbits, with each bit corresponding to a heating element, and thegeneration of the second part includes a second plurality of bits, thesum of which defines a timing pattern; in which the selection of thefirst peak power includes engaging each heating element in response tothe corresponding bit in the digital word first part; and in which thefirst duration of the first peak power includes engaging heatingelements for a duration responsive to the sum of the bits in the digitalword second part.
 42. The method of claim 40 in which the selection ofthe peak power includes the difference between the first and second peakpower being less than 200 watts.